1. Field of the Invention
The present invention relates to a tracking servo system of optical disk player.
2. Description of Background Information
In an optical disk player for playing disk recording medium in the form of a disk such as video disk or digital audio disk, etc. (hereinafter simply referred as "disk"), a tracking servo system for controlling the accurate tracing of the recording track is indispensable for positioning the information reading light spot of the pickup so that it can accurately trace the recording track irrespectively of the eccentricity of the disk.
This tracking servo system is designed as a so-called closed loop servo, which generates a tracking error signal corresponding to the deviation of the information reading light spot in the radial direction of the disk with respect to the recording track of the disk. By driving the tracking actuator for shifting the information reading light spot in the radial direction of the disk according to the tracking error signal, the position of the information reading light spot is controlled relative to the recording tracking.
During the so-called jumping operation, in which the information reading light spot jumps over the recording track, an acceleration signal of a polarity corresponding to the jumping direction is supplied to the tracking actuator with the servo loop in open status. After giving a braking force of a certain level by supplying a deceleration signal having polarity opposite to the acceleration signal at zero-cross timing of the level of tracking error signal during jumping operation, the servo is lead-in by rendering the servo loop in the closed state.
As the method for generating the tracking error signal, the three-beam method, push-pull method, time difference detection method, etc. are known. Of these methods, description is made here for the principle of the time difference detection method, referring to FIG. 1. First, as the photodetector to receive reflection light beam from the disk, which is incorporated in the pickup, a so-called quadrant type photodetector 51 is used, which consists of 4 photoelectric conversion elements 51a-51d arranged in such manner that the light receiving plane is divided into 4 parts by a division line L.sub.1 along tangential line of the track and a division line L.sub.2 perpendicular to it. This photodetector 51 is disposed in such manner that the center 0 of the light receiving plane coincides with the optical axis of the reflection light beam from the disk D when the tracking status is adequate. Of these 4 photoelectric conversion elements 51a-51d, the outputs Sa and Sc of the photoelectric conversion elements 51a and 51c arranged on a diagonal line and the outputs Sb and Sd of the photoelectric conversion elements 51b and 51d are added by the adders 52 and 53 respectively. The summing outputs (Sa+Sc) and (Sb+Sd) pass through band pas filters (BPF) 54 and 55 and limiters (LIM) 56 and 57 and are sent to a phase comparator 58. The phase comparator 58 issues a voltage proportional to the phase difference of two outputs as the tracking error signal. The total sum of the outputs as the tracking error signal. The total sum of 51a-51d is delivered as a reading RF signal.
As described above, of the phase difference generated between the outputs of 4 photoelectric conversion elements 51a-51d, a phase variation component, changing according to the deviation of the information reading beam spot relative to the recording track in the radial direction of the disk, is detected and the tracking error signal is generated by the phase variation component. This time difference detection method is already known (Reference is directed to Japanese Patent application Laid Open No. 57-181433). The method has an advantage that it is hardly adversely affected by the deviation of intensity distribution of reflection light beam from the disk. On the other hand, in the so-called off track position where the information reading beam spot is located between the recording tracks, by using RF component for detection, the RF component of the adjacent track leaks, and correct error signal will not be obtained. As the result, the zero-cross timing of the tracking error signal level becomes unstable. In such a case, the acceleration period cannot be determined, so that the reliability of jump algorithm is lowered when the time difference detection method is used to generate the tracking error signal.